The present invention relates to a rotatable hollow cylinder, particularly useful as a drying cylinder for a paper machine.
Typically, such a cylinder is heated with a condensable fluid. On the cylindrical inner surface of the cylinder, there is an array of at least two moldings or cleats extending in the axial direction of the cylinder, which are fixed in boreholes provided in the inner surface of the wall of the cylinder. Such cleats or moldings are provided in drying cylinders because they increase the turbulance of the condensate that collects in a cylindrical layer about the inner surface of the cylinder at high peripheral speeds. Thus the heat transfer into the cylinder wall is improved.
In German Provisional Patent (Auslegeschrift) No. 23 03 334 and corresponding U.S. Pat. No. 3,808,700, cleats or moldings are fastened at the centers by screws located in threaded holes in the cylinder wall and the ends of which engage in each other. If a screw should happen to loosen, the entire cleat assembly becomes loose and the cleats might turn around their fastening screw. Furthermore, the deep threaded holes impair the mechanical strength of the cylinder wall. Additionally, producing threaded holes in the wall of the cylinder is a time-consuming operation.
To avoid the foregoing disadvantages, German Provisional Patent Auslegeschrift No. 23 30 199 suggests fastening the cleats or moldings merely by connecting them to each other by means of spring bars. This takes up a large amount of space within the cylinder and also results in difficulties in maintaining the cleats or moldings at the same distance apart on the inner surface of the cylinder.